Typical propulsion systems for ships have a single spiral propeller. However, the propulsion system equipped with one propeller cannot use rotational energy of a water stream caused by rotation of the propeller as a propulsive force, and thus suffers a great loss of energy.
Contra-rotating propellers (CRP) can recover the rotational energy lost in this way as the propulsive force. In the CRP, two propellers installed on the same axis generate the propulsive force while rotating in opposite directions. The rear propeller recovers the rotational energy of a fluid passing through the front propeller as the propulsive force while rotating reversely. Thus, the CRP can exert higher propulsion performance than the propulsion system equipped with one propeller.
However, the CRP includes a contra-rotating unit for implementing contra-rotation of the two propellers, a hollow shaft, and so on. For this reason, it is relatively difficult to manufacture and install the CRP. The CRP requires a high-tech level in order to maintain reliability and ensure stable operation.
A contra-rotating propulsion system equipped with a planetary gear type contra-rotating unit and a hollow shaft installed in a hull is disclosed in US Patent Application Publication No. 2011/0033296 (entitled CONTRA-ROTATING PROPELLER MARINE PROPULSION DEVICE).